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Charging Li-ion cells in series

M

Mark Jones

Jan 1, 1970
0
Hi. I'm looking at the LM3622 datasheet
(http://www.national.com/ds/LM/LM3622.pdf). It's a simple Li-ion
linear charger. Comes in 4v/8v Li-ion versions. In Li-ion topology, a
lot of fuss is made about the cell's final voltage accuracy and how
overshooting this will destroy the cell. But what would happen if two
cells in series were chargeed with one unequally depleted? Wouldn't
the other cell shoot way over its rated voltage, destroying it? It
makes no sense that nowhere has this been mentioned. Is this a real
concern? Should the two Li-ion cells in series be equalized when
charging or charged separately?

TIA.
 
J

Joerg

Jan 1, 1970
0
Hi Mark,

For a brief but concise summary see Atmel's app note AVR450. It's about
a uC based charger but explains charging methods of the different
chemistry batteries.

Be careful since Li Ion batteries can explode when mistreated. As far as
I know it is not a good idea to charge in a series configuration.

Regards, Joerg
 
I

Ian Stirling

Jan 1, 1970
0
Mark Jones said:
Hi. I'm looking at the LM3622 datasheet
(http://www.national.com/ds/LM/LM3622.pdf). It's a simple Li-ion
linear charger. Comes in 4v/8v Li-ion versions. In Li-ion topology, a
lot of fuss is made about the cell's final voltage accuracy and how
overshooting this will destroy the cell. But what would happen if two
cells in series were chargeed with one unequally depleted? Wouldn't
the other cell shoot way over its rated voltage, destroying it? It

Yes.
There are various ways to get round this.
For example, when on discharge you detect an imabalance, you connect
a small load to the unbalanced cell.
Or, do the same on charge, to shunt some of the charge current away.
 
T

Tim Wescott

Jan 1, 1970
0
Joerg said:
Hi Mark,

For a brief but concise summary see Atmel's app note AVR450. It's about
a uC based charger but explains charging methods of the different
chemistry batteries.

Be careful since Li Ion batteries can explode when mistreated. As far as
I know it is not a good idea to charge in a series configuration.

Regards, Joerg

Li-Ion batteries have been taken up by the electric-powered model
airplane crowd, because of their good energy density characteristics.
They are usually pushed _hard_; discharged fast, charged in series, etc.

So now there's a little ceramic vault you can buy to charge and store
your Li-Ion e-flight batteries in, to contain the fire...
 
J

Joop

Jan 1, 1970
0
Mark Jones said:
Hi. I'm looking at the LM3622 datasheet
(http://www.national.com/ds/LM/LM3622.pdf). It's a simple Li-ion
linear charger. Comes in 4v/8v Li-ion versions. In Li-ion topology, a
lot of fuss is made about the cell's final voltage accuracy and how
overshooting this will destroy the cell. But what would happen if two
cells in series were chargeed with one unequally depleted? Wouldn't
the other cell shoot way over its rated voltage, destroying it? It
makes no sense that nowhere has this been mentioned. Is this a real
concern? Should the two Li-ion cells in series be equalized when
charging or charged separately?

TIA.

You should take it into account. In one of my hobby project I use 2
cells taken from a used laptop pack in series. These are (slow)
charged with a simple 78L08. I put a 1K resistor parallel to each cell
during charging.
Cell are thus not fully charged (4 instead of 4.2V) which gives me a
margin of inbalance that will also be corrected up to 4mA by the
shunt.
There is some lost capacity due to the 4V, but it is minimal.
Worked for me, simple and cheap.

Joop
 
M

Mark Jones

Jan 1, 1970
0
Joop said:
You should take it into account. In one of my hobby project I use 2
cells taken from a used laptop pack in series. These are (slow)
charged with a simple 78L08. I put a 1K resistor parallel to each cell
during charging.
Cell are thus not fully charged (4 instead of 4.2V) which gives me a
margin of inbalance that will also be corrected up to 4mA by the
shunt.
There is some lost capacity due to the 4V, but it is minimal.
Worked for me, simple and cheap.

Joop


Thanks everyone for the responses. It's amazing that no datasheet
mentions this posibility. Then again, we are supposed to know this,
right? :)

Here's an interesting equalization technique, wonder if it will work:


Mark Jones .-------------+--------+--> Vdd

Helios Studios ||--' EM MOSFET | |
.--------. ||<-. x4 | |
| Q |-----+---||--+ | |
| | | | | 10uF | +4.1v
| Flip | | +-----. --- -----
| Flop | | | | --- ---
| 10kHz | | ||--' | | | -
| _ | | ||<-. | | |
| Q |--+--|---||--+ | | |
'--------' | | | | | |
| | +-----|-------+--------+
| | | --- | |
| | ||--' --- | |
| | ||<-. | 470uF | |
| +---||--+ | | 10uF | +4.1v
| | | --- -----
| +-----' --- ---
| | | | -
| ||--' | |
| ||<-. | |
'------||--+ | |
'-------------+--------+--> Vee


-M
 
M

mike

Jan 1, 1970
0
Joop said:
You should take it into account. In one of my hobby project I use 2
cells taken from a used laptop pack in series. These are (slow)
charged with a simple 78L08. I put a 1K resistor parallel to each cell
during charging.
Cell are thus not fully charged (4 instead of 4.2V) which gives me a
margin of inbalance that will also be corrected up to 4mA by the
shunt.
There is some lost capacity due to the 4V, but it is minimal.
Worked for me, simple and cheap.

Joop

I've thought about doing this, but couldn't figger out how to make it
happen automatically. Cells are in a PDA, so while I have access to the
center point, don't have access to much else to control it.
What method did you use to switch in the resistors?

Thanks, mike

--
Return address is VALID.
500MHz Tek DSOscilloscope TDS540 $2200
http://nm7u.tripod.com/homepage/te.html
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
http://www.geocities.com/SiliconValley/Monitor/4710/
 
B

budgie

Jan 1, 1970
0
Hi. I'm looking at the LM3622 datasheet
(http://www.national.com/ds/LM/LM3622.pdf). It's a simple Li-ion
linear charger. Comes in 4v/8v Li-ion versions. In Li-ion topology, a
lot of fuss is made about the cell's final voltage accuracy and how
overshooting this will destroy the cell. But what would happen if two
cells in series were chargeed with one unequally depleted? Wouldn't
the other cell shoot way over its rated voltage, destroying it? It
makes no sense that nowhere has this been mentioned. Is this a real
concern? Should the two Li-ion cells in series be equalized when
charging or charged separately?

Most commercial multi-cell packs have a pack protection module. Among its
functions are detecting under/over voltage (of the series string as a whole),
over-current on discharge, and cell_voltage_imbalance.

The module intervenes by effectively turning off a FET in series with the cells,
thus preventing any significant current flow in or out. Cell imbalance is one
of the causes of pack "failure" in laptops.
 
J

Joop

Jan 1, 1970
0
mike said:
I've thought about doing this, but couldn't figger out how to make it
happen automatically. Cells are in a PDA, so while I have access to the
center point, don't have access to much else to control it.
What method did you use to switch in the resistors?

Thanks, mike
With a switch (dual pole/make). The GND is always connected to the
cells. The cells center tap is switched in to the center between the
resistors. The V+ is switched to the +8V of the 78L08 and the actual
circuit.

The 78L08 seems to handle the 8V without noticable current draw. The
shunt takes 4mA of course. But my circuit uses about 50mA so I found
the added 4 mA neglectable.

Charging only happens when I plug in the transformer. It could all be
done a bit nicer on all fronts but it works with affordable
of-the-shelf components.
My device is an LC meter which I do not use a very often. The li-ion
makes sure that it always works because the self discharge is
extremely low. A potential reduced capacity of the old cells, the sub
optimal charge (4V) and the waist current (4mA) are all unimportant
compared to full enough cells whenever I need the meter for a couple
of minutes.

As for your PDA, do you really have multiple cells in there? Most
mobile phones e.g. just have a single cell for simplicity. The are
already enough rumours out there about exploded non-brand cells. Any
explosion due to unsafe charge circuits with aging cells put in series
is not what manufacturers would like to be linked to. They just use a
dc/dc converter if needed at all to avoid the issue.
For laptops the high power draw makes it more efficient to use a
higher pack voltage. But then you see 3 of 4 cells in series and
complex circuitry to safeguard the pack as well as provide SMB
communication with the computer.

Joop
 
J

Joop

Jan 1, 1970
0
With a switch (dual pole/make). The GND is always connected to the
cells. The cells center tap is switched in to the center between the
resistors. The V+ is switched to the +8V of the 78L08 and the actual
circuit.
Sorry, I got carried away. To come back to your question, why not use
a small relay to switch in the top and center cell connection whenever
the mains power is available?

Joop
 
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